Happened to me before.

Was selling something in RMAH for $250 and a while after I received the email saying that my item was sold, my account got locked and all my items in RMAH was returned back to stash.

Sent them am email but didn't want to wait days for their reply so I called them using Skype and they unbanned my account. Expect a hard time calling them and to be put on hold for hours, I had to wait on the line for 2 hours or so.

For my case it was because the buyer was not using a legit way to buy the item, maybe stolen/hacked paypal.